For 循环语句
基础知识
for循环可以遍历任何序列的项目，如一个列表或者一个字符串。
语法：
for 循环规则：
do sth
1 >>> for i in "python" : #用i这个变量遍历这个字符串的每一个字符 2 ... print i #将遍历的字符打印出来 3 ... 4 p 5 y 6 t 7 h 8 o 9 n 10 >>> lst =["baidu","google","ali"] 11 >>> for i in lst: #用变量i遍历这个列表，将每个元素打印出来 12 ... print i 13 ... 14 baidu 15 google 16 ali 17 >>> t =tuple(lst) 18 >>> t 19 (‘baidu‘, ‘google‘, ‘ali‘) 20 >>> for i in t: #用变量i遍历元组，将每个元素打印出来 21 ... print i 22 ... 23 baidu 24 google 25 ali 26 >>> d =dict([("lang","python"),("website","baidu"),("city","beijing")]) 27 >>> d 28 {‘lang‘: ‘python‘, ‘website‘: ‘baidu‘, ‘city‘: ‘beijing‘} 29 >>> for k in d: #用变量k遍历这个字典，将每个key打印出来 30 ... print k 31 ... 32 lang 33 website 34 city 35 >>> for k in d: #用变量k遍历字典d 36 ... print k,"-->",d[k] #将key值和value值打印出来 37 ... 38 lang --> python 39 website --> baidu 40 city --> beijing 41 >>> d.items() #以列表返回可遍历的(键, 值) 元组 42 [(‘lang‘, ‘python‘), (‘website‘, ‘baidu‘), (‘city‘, ‘beijing‘)] 43 >>> for k,v in d.items(): #用key value遍历d.items()的元组列表 44 ... print k,"-->",v #取得key ,value 45 ... 46 lang --> python 47 website --> baidu 48 city --> beijing 49 >>> for k,v in d.iteritems(): iteritems 返回的是迭代器 推荐使用这个 50 ... print k,v 51 ... 52 lang python 53 website baidu 54 city beijing 55 >>> d.itervalues() 返回的是迭代器 56 <dictionary-valueiterator object at 0x0000000002C17EA8> 57 >>>
判断对象是否可迭代
1 >>> import collections #引入标准库 2 >>> isinstance(321,collections.Iterable) #返回false,不可迭代 3 False 4 >>> isinstance([1,2.3],collections.Iterable) #返回true,可迭代 5 True
1 >>> l =[1,2,3,4,5,6,7,8,9] 2 >>> l[4:] 3 [5, 6, 7, 8, 9] 4 >>> for i in l[4:]: #遍历4以后的元素 5 ... print i 6 ... 7 5 8 6 9 7 10 8 11 9 12 >>> help(range) #函数可创建一个整数列表，一般用在 for 循环中 13 Help on built-in function range in module __builtin__: 14 15 range(...) 16 range(stop) -> list of integers 17 range(start, stop[, step]) -> list of integers #计数从 start 开始,计数到 stop 结束，但不包括 stop,step：步长，默认为1 18 19 Return a list containing an arithmetic progression of integers. 20 range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0. 21 When step is given, it specifies the increment (or decrement). 22 For example, range(4) returns [0, 1, 2, 3]. The end point is omitted! 23 These are exactly the valid indices for a list of 4 elements. 24 25 >>> range(9) 26 [0, 1, 2, 3, 4, 5, 6, 7, 8] 27 >>> range(2,8) 28 [2, 3, 4, 5, 6, 7] 29 >>> range(1,9,3) 30 [1, 4, 7] 31 >>> l 32 [1, 2, 3, 4, 5, 6, 7, 8, 9] 33 >>> range(0,9,2) 34 [0, 2, 4, 6, 8] 35 >>> for i in range(0,9,2): 36 ... print i 37 ... 38 0 39 2 40 4 41 6 42 8 43 >>>
1 #! /usr/bin/env python 2 #coding:utf-8 3 4 aliquot =[] #创建一个空的列表 5 6 for n in range(1,100): #遍历1到100 的整数 7 if n %3==0: #如果被3整除 8 aliquot.append(n) #将n值添加到列表中 9 10 print aliquot
zip() 函数
函数用于将可迭代的对象作为参数，将对象中对应的元素打包成一个个元组，然后返回由这些元组组成的列表。
如果各个迭代器的元素个数不一致，则返回列表长度与最短的对象相同，利用 * 号操作符，可以将元组解压为列表。
返回一个列表，这列表是以元组为元素
1 >>> a =[1,2,3,4,5] 2 >>> b =[9,8,7,6,5] 3 >>> c =[] 4 >>> for i in range(len(a)): 5 ... c.append(a[i]+b[i]) 6 >>> for i in range(len(a)): 7 ... c.append(a[i]+b[i]) 8 ... 9 >>> c 10 [10, 10, 10, 10, 10] 11 >>> help(zip) 12 Help on built-in function zip in module __builtin__: 13 14 zip(...) 15 zip(seq1 [, seq2 [...]]) -> [(seq1[0], seq2[0] ...), (...)] 16 17 Return a list of tuples, where each tuple contains the i-th element 18 from each of the argument sequences. The returned list is truncated 19 in length to the length of the shortest argument sequence. 20 21 >>> a 22 [1, 2, 3, 4, 5] 23 >>> b 24 [9, 8, 7, 6, 5] 25 >>> zip(a,b) 26 [(1, 9), (2, 8), (3, 7), (4, 6), (5, 5)] 27 >>> c =[1,2,3] 28 >>> zip(c,b) 29 [(1, 9), (2, 8), (3, 7)] 30 >>> zip(a,b,c) 31 [(1, 9, 1), (2, 8, 2), (3, 7, 3)] 32 >>> d=[] 33 >>> for x,y in zip(a,b): 34 ... d.append(x+y) 35 ... 36 >>> d 37 [10, 10, 10, 10, 10] 38 >>> r =[(1,2),(3,4),(5,6),(7,8)] 39 >>> zip(*r) 40 [(1, 3, 5, 7), (2, 4, 6, 8)] 41 >>>
enumerate()函数
函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标，一般用在 for 循环当中。
语法：
enumerate(sequence, [start=0])
sequence -- 一个序列、迭代器或其他支持迭代对象
start -- 下标起始位置。
返回值： enumerate枚举对象
1 >>> help(enumerate) 2 Help on class enumerate in module __builtin__: 3 4 class enumerate(object) 5 | enumerate(iterable[, start]) -> iterator for index, value of iterable 6 | 7 | Return an enumerate object. iterable must be another object that supports 8 | iteration. The enumerate object yields pairs containing a count (from 9 | start, which defaults to zero) and a value yielded by the iterable argument. 10 | enumerate is useful for obtaining an indexed list: 11 | (0, seq[0]), (1, seq[1]), (2, seq[2]), ... 12 | 13 | Methods defined here: 14 | 15 | __getattribute__(...) 16 | x.__getattribute__(‘name‘) <==> x.name 17 | 18 | __iter__(...) 19 | x.__iter__() <==> iter(x) 20 | 21 | next(...) 22 | x.next() -> the next value, or raise StopIteration 23 | 24 | ---------------------------------------------------------------------- 25 | Data and other attributes defined here: 26 | 27 | __new__ = <built-in method __new__ of type object> 28 | T.__new__(S, ...) -> a new object with type S, a subtype of T 29 30 >>> weeks =["sun","mon","tue","web","tue","fri","sta"] 31 >>> for i,day in enumerate(weeks): 32 ... print str(i)+":"+day 33 ... 34 0:sun 35 1:mon 36 2:tue 37 3:web 38 4:tue 39 5:fri 40 6:sta 41 >>> for i in range(len(weeks)): 42 ... print str(i)+":"+weeks[i] 43 ... 44 0:sun 45 1:mon 46 2:tue 47 3:web 48 4:tue 49 5:fri 50 6:sta 51 >>> raw ="Do you love canglaoshi? canglaoshi is a good teacher." 52 >>> raw_lst =raw.split(" ") 53 >>> raw_lst 54 [‘Do‘, ‘you‘, ‘love‘, ‘canglaoshi?‘, ‘canglaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘] 55 >>> for i,w in enumerate(raw_lst): 56 ... if w =="canglaoshi": 57 ... raw_lst[i]="luolaoshi" 58 ... 59 >>> raw_lst 60 [‘Do‘, ‘you‘, ‘love‘, ‘canglaoshi?‘, ‘luolaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘] 61 >>> for i,w in enumerate(raw_lst): 62 ... if "canglaoshi" in w: 63 ... raw_lst[i]="luolaoshi" 64 ... 65 >>> raw_lst 66 [‘Do‘, ‘you‘, ‘love‘, ‘luolaoshi‘, ‘luolaoshi‘, ‘is‘, ‘a‘, ‘good‘, ‘teacher.‘] 67 >>> a =range(10) 68 >>> a 69 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 70 >>> s =[] 71 >>> for i in a: 72 ... s.append(i*i) 73 ... 74 >>> s 75 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] 76 >>> b = [i*i for i in a] #列表解析 77 >>> b 78 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] 79 >>> c = [i*i for i in a if i%3==0] #列表解析，加入限制条件 80 >>> c 81 [0, 9, 36, 81] 82 >>>
列表解析